Ugh, I LOVE THAT MOVIE. And it's not that they started to die, Jesus. It was sooooo deep They were God's angels, supernatural beings made in pairs. Basically, when they together they powers would start to diminish until they felt mortal feeling and pain. Would they risk it all to be in love and possibly die or experience pain? Or would they be selfless and move on and protect life. Will Smith's character was made to fight/protect so he was always sacrificing himself to save her.